This counter is made up of 4 master-slave JK flip-flops (The design for the master-slave JK flip-flop is located here: http://everycircuit.com/circuit/5188452372447232). Master-slave flip-flops are useful because they do not oscillate at all when the clock is true. The Q' outputs lead to the clocks of the next flip-flop. The Q' outputs also act as the output of the counter.
